Astronomy Dictionary

H H is an asteroid's absolute magnitude, the visual magnitude the asteroid would have if it were located 1 AU from Earth.

HADLEY, JOHN H. John Hadley (1682-1744) was an English mathematician and inventor who built the first reflecting telescope and invented an improved quadrant (known as Hadley's quadrant).

HADLEY RILLE Hadley Rille is a long valley on the surface of the moon. This rille is 75 miles (125 km) long, 1300 feet (400 m) deep, and almost 1 mile (1500 m) wide at its widest point. It was formed by molten basaltic lava that carved out a steep channel along the base of the Apennine Front (which was explored by the Apollo 15 astronauts in 1971).

HALE-BOPP COMET Hale Bopp (designated C/1995 O1) is a periodic comet (made of frozen gas and dust), that orbits around the sun. Its earliest-recorded sighting was on July 23, 1995. It was independently discovered by Alan Hale (of New Mexico) and Thomas Bopp (of Arizona), both amateur astronomers. This comet has a diameter of about 40 km; it's rotation rate is 11.4 hours. It was last seen in 1997 and will be seen next in the year 4377; its period is 2380 years.

HALE, GEORGE E. George Ellery Hale (June 9, 1868 - February 21, 1938) was an astronomer who founded the Yerkes Observatory (1892), the Mt. Wilson Observatory (in 1904), and the Palomar Observatory. Hale invented the spectroheliograph (a device used to analyse the Sun's spectrum) when he was an undergraduate at the Massachusetts Institute of Technology (Cambridge, Massachusetts, USA). Later, Hale discovered that sunspots were low-temperature areas on the sun and that they had high magnetic fields.

HALF-LIFE The half-life of a radioisotope is the amount of time it takes for half of the radioisotope to decay.

HALF MOON A half moon looks like half a circle. It is sometimes called a quarter moon (this moon has completed one quarter of an orbit around the Earth from its full or new position, and one quarter of the moon's surface is visible from Earth).

HALLEY'S COMET Halley's comet is a periodic comet (made of frozen gas and dust), that orbits around the sun. Its earliest-recorded sighting was in 240 B.C. in China, but Edmund Halley was the first person to recognize that it was periodic. It was last seen in 1986 and will be seen next in the year 2061; its period is 76 years. When the Earth passes through Halley's comet's orbit (twice each year), its detritus causes the meteor showers the Eta Aquarids and the Orionids.

HALO A halo is a luminous ring that is sometimes seen surrounding the sun or the moon. Some parts of the halo are very bright, others are not very bright. Sometimes, only a part of the ring is visible. The halo is produced as light is reflected and refracted through tiny, flat ice crystals in the atmosphere. Halos are always an angle of 22° away from the sun or moon, due to the hexagonal structure of the ice crystals. The diameter of the halo is about an eighth of the sky.

HARTMANN, JOHANNES F. Johannes Franz Hartmann (1865-1936) was a German astrophysicist who, in 1904, discovered clouds of interstellar calcium gas (he detected the absorption lines of ionized calcium atoms using spectrography while studying binary stars). He also developed a theory about novas, studied the asteroid #433 (Eros) and developed a method of testing telescope lenses, which is still named for him.

HARVEST MOON The Harvest moon is the full moon that appears closest in time to the Autumnal Equinox, occurring in late September or early October.

HELENE Helene is one of the smaller of the 18 moons of Saturn. Helene has a radius of about 18x16x15 km and is irregularly shaped. Helene orbits at about 377,400 km from Saturn, leading Dione by 60°. Its orbital period is 2.74 days. Helene was discovered by Discovered: P. Laques & J. Lecacheus in 1980.

HELIOCENTRIC A Heliocentric system is one in which a sun is at the center.

HELIOCENTRIC SYSTEM In the heliocentric model of the solar system, all the planets orbit around the Sun.

HELIOPAUSE The heliopause is the boundary of the heliosphere of the Sun in which the solar wind's density decreases greatly (and its speed also declines). The location of this transition region is unknown, but must be at more than 50 AU from the Sun.

HELMHOLTZ, HERMANN VON Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was a German astrophysicist who studied solar energy production and star formation. See Helmholtz contraction.

HEMISPHERE A hemisphere is half of a sphere.

HENRY DRAPER CATALOG The Henry Draper catalog is a catalog of stars in which every star is classified by its stellar spectrum. This sytem is named for the astronomer Henry Draper, but was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall. For example, the star Vega is HD 172167 (the spectral type is not in the HD number).

HERCULES Hercules is a Northern Hemisphere constellation that is the fifth largest in the sky. It is named for Hercules, the legendary hero of Greek mythology. The brightest of its rather dim stars is Ras Algethi (alpha Her), a variable red supergiant. The four stars of the central trapezoid within Hercules, epsilon Her, zeta Her, eta Her, and pi Her, form the asterism called Keystone. The globular star cluster M13 is located on the western part of the Keystone. The Tau Herculid meteor shower seems to radiate from Hercules.

HERSCHEL, WILLIAM Sir William Herschel (1738-1822) was a British astronomer and organist who built an improved reflecting telescope and used it to discover the planet Uranus (March 13, 1781) and moons of Uranus and of Saturn. Herschel cataloged over 2500 discoveries, mostly deepsky objects. Herschel's sister Caroline Lucretia Herschel (1750-1848) helped him in his discoveries and discovered many clusters and nebulae (and 8 comets) herself.

HERTZSPRUNG, EJNAR Ejnar Hertzsprung (1873-1967) was a Danish astronomer who, independently of H. N. Russell, realized the relationship between a star's temperature (color) and its brightness, and designed a diagram illustrating this relationship in 1911, later called the Hertzsprung-Russell Diagram.

HERTZSPRUNG-RUSSELL DIAGRAM The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3 very different types of stars: Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting hydrogen into helium. For these stars, the hotter they are, the brighter. Giants and Supergiants are above the main sequence. Faint white dwarfs (hot, dim stars) are below the main sequence stars.

HEVELIUS, JOHANNES Johannes Hevelius (1611-1687) was a Polish astronomer who published the first moon map. He also published a celestial atlas introducing many constellations (including Canes Venatici, Lacerta, Lynx, Sextans, etc.).

HIGH TIDE High tide is the time of high water. High tides occur when the gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon.

HIMALIA Himalia is Jupiter's tenth moon. Himalia is 110 miles (170 km) in diameter and orbits 7,000,000 miles (11,480,000 km) from Jupiter. Himalia has a mass of 9.5 x 1018kg. It orbits Jupiter in 250.5662 (Earth) days. Very little is known about Himalia. Himalia was discovered by C. Perrine in 1904.

HIPPARCHUS Hipparchus (190-120 B.C.) was an ancient Greek astronomer who compiled first-known catalogue of stars and first map of the skies. He listed 850-1,000 stars, organized by constellation. He noted each star's position and brightness (he rated the brightness on a scale from 1 to 6, the brightest being 1). Hipparchus also devised the system of epicycles, an Earth-centered system in all celestial objects moved in perfect circles around the Earth. He also founded trigonometry.

HORIZON The horizon is an imaginary circle that delimits the sky and the Earth, or an extension of the plane of the observer (at an altitude of 0 degrees).

HORIZONTAL BRANCH The horizontal branch is a part of the Hertzsprung -Russell (H-R) diagram that represents stars that burn helium in thier cores. These mostly large stars lie along the top of the H-R diagram.

HORSEHEAD NEBULA The Horsehead Nebula is a huge cloud of dust in space that is shaped like a horse's head. It is luminous at its edges because it is in front of a bright emission nebula. It is located in the constellation called Orion.

H-R DIAGRAM The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars' color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3 very different types of stars: Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting hydrogen into helium. For these stars, the hotter they are, the brighter. Giants and Supergiants are above the main sequence. Faint white dwarfs (hot, dim stars) are below the main sequence stars.

HST HST is an abbreviation for the Hubble Space Telescope, a powerful telescope in orbit around the Earth. HST takes pictures and spectra of objects in space without the interference of the atmosphere (which makes telescopic images from the ground have less detail); it transmits the pictures and spectra back to scientists on Earth. The telescope was launched into space in April, 1990, and was repaired in December, 1993. It was named for the American astronomer Edwin Hubble.

HUBBLE CONSTANT The Hubble Constant, H0 is the number (not actually a constant) which shows the rate at which the universe is expanding. It determines the relationship between how far a galaxy is from us and how fast it is receding from us (because of the expansion of the Universe). H0 is between 50 and 100 km/s/mparsec. Hubble's constant can be used to estimate the size and age of the Universe (Hubble Time). It was formulated by E. P. Hubble in 1925.

HUBBLE SPACE TELESCOPE The Hubble Space Telescope (or HST) is a powerful telescope in orbit around the Earth. HST transmits pictures and spectra of objects in space without the interference of the atmosphere (which makes telescopic images from the ground have less detail). It was launched into space in April 1990 and was repaired in December, 1993. It was named for the American astronomer Edwin Hubble.

HUBBLE TIME Hubble Time is an estimate of the age of the universe; it is the inverse of the Hubble constant.

HUGGINS, WILLIAM and MARGARET Sir William Huggins (February 7, 1824-May 12, 1910) was an amateur English astronomer who was the first person to use spectroscopy to determine the compositions of astronomical objects (in 1861). He determined that the Sun and the stars are composed mostly of the element hydrogen. He also examined the spectra of nebulae and comets. Huggins' wife (they were married in 1875), Margaret Lindsay Murray Huggins (1848-1915), was a self-taught astronomer who did extensive work in spectroscopy and photography. Margaret studied the Orion Nebula extensively. William and Margaret were the first people to realize that some nebulae, like the Orion Nebula, consisted of amorphous gases (and were not a congregation of stars, like the nebula Andromeda). A lunar crater, a Martian crater, and an asteroid (#2635 Huggins) have been named for William Huggins.

HUYGENS, CHRISTIAN Christian Huygens (1629-1695) was a Dutch physicist and astronomer who developed new methods for grinding and polishing glass telescope lenses (about 1654). With his new, powerful telescopes, he identified Saturn's rings and discovered Titan, the largest moon of Saturn in 1655. Huygens also invented the pendulum clock in 1656 (eliminating springs), wrote the first work on the calculus of probability (De Ratiociniis in Ludo Aleae, 1655), and proposed the wave theory of light (Traité de la lumiere, 1678).

HYBRID ECLIPSE A hybrid eclipse is an annular solar eclipse in which a brief total eclipse occurs in a small region along the eclipse's central path.

HYDROGEN Hydrogen is the element with the atomic number 1. It is the lightest element and the most abundant in the universe. Its nucleus is a single proton which is orbited by one electron. It fuels nuclear fusion that occurs within stars, converting hydrogen into helium. The sun is 75% hydrogen.

HYDROLOGIC CYCLE The Hydrologic Cycle (also known as the water cycle) is the journey water takes as it circulates from the Earth to the sky and back again.

HYPATIA Hypatia of Alexandria (AD 370(?)-415) was a Greek mathematician, astronomer, teacher, and head of the Platonist school at Alexandria about AD 400. Hypatia wrote commentaries on the astronomical canon of Ptolemy and did work on conic sections . Her works are lost, but are referred to in the Suda lexicon. She was the daughter of the mathematician and philosopher Theon of Alexandria (he was also the last head of the Museum at Alexandria). A pagan, she was murdered in 415 by Christian monks in a religious/political struggle. The lunar Crater Hypatia and Rimae Hypatia were named for her.

HYPERBOLA A hyperbola is a conic section (the intersection of a cone with a plane) that has two mirror-image branches. Hyperbolas have an eccentricity greater than 1.

HYPERBOLIC ORBIT A hyperbolic orbit is an orbit in which the eccentricity is greater than 1.

HYPERION Hyperion is one of the 18 moons of Saturn. Hyperion is the largest-known irregular-shaped body in our Solar System. It has a variable rotational period. Craters mark the surface. Iapetus has a radius of about 180x140x112.5 km and is irregularly shaped. Hyperion orbits at about 1,481,100 km from Saturn. Its orbital period is 21.27 days. It was discovered by W. & G. Bond/W. Lassell in 1848.

Astronomy Dictionary

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